What is a possible reason for orange made from red and yellow appearing muddy?

Correct answer: Blue contamination on the brush

The governing concept is pigment contamination during colour mixing. Orange is made from red and yellow, but blue residue on a brush introduces an unwanted third pigment. Because blue is complementary to orange, even a small amount can reduce saturation and make the mixture look dull or muddy. Red and yellow are required ingredients, and a clean palette would not cause this problem. Hence option A is correct.